Delta-sigma modulator, method of switching delta-sigma modulator, and digital amplifier

ABSTRACT

A delta-sigma modulator includes a quantization bit rate detection unit that detects a quantization bit rate of a digital audio signal, a volume setting value detection unit that detects a volume setting value of the digital audio signal; a filter that includes plural sets of filter coefficients having different shaping properties and allows the quantization noise to pass, and a filter coefficient switcher that switches the filter coefficients of the filter in accordance with the detection results of the quantization bit rate detection unit and the volume setting value detection unit. The delta-sigma modulator detects the quantization bit rate of an input signal and switches the filter coefficients to the low-order filter coefficient set when a source has a low quantization bit rate.

[0001] The present disclosure relates to the subject matter contained inJapanese Patent Application No.2002-340745 filed Nov. 25, 2002, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a delta-sigma modulator, amethod of switching a delta-sigma modulator, and a digital amplifier.

[0004] 2. Description of the Related Art

[0005] Conventionally, a digital amplifier using delta-sigma modulationshifts quantization noise to a high-frequency band by noise shaping, tothereby secure the S/N ratio of an audio band.

[0006]FIG. 1 is a block diagram showing the configuration of aconventional quintic delta-sigma modulator. Noise shaping filtercoefficients (a1, a2, a3, a4, a5) are fixed. In this manner, thecoefficients of the conventional delta-sigma modulator serving as anoise shaping filter used in a digital amplifier are fixed (e.g., see J.M. Goldberg and A. B. Sandler, “Noise Shaping and Pulse-Width Modulationfor an All-Digital Audio Power Amplifier”, Journal of the AudioEngineering Society, Vol. 39, No. 6, 1991.).

[0007] It should be noted that, although the coefficients of thedelta-sigma modulator are fixed in the above-described prior art, thereis a modulator that selects the filter coefficients in accordance withan input signal level with the purpose of making the amplitude frequencyproperties of the quantization noise coincide with the auditorysensitivity properties of humans, and the configuration thereof is shownin FIG. 2 (e.g., see JP-A-7-15281, page 3 and FIG. 1).

[0008] In the prior art shown in FIG. 2, the level of an m-bit inputsignal is detected by a level detection unit 103, and the result ofdetermination by a level determination unit 104 is input to a filtercoefficient storage memory 105. Filter coefficients that make theamplitude frequency properties of quantization noise coincide withauditory sensitivity properties changing in accordance with the level ofthe m-bit input signal is stored in advance in the filter coefficientstorage memory 105. Properties of a variable filter 106 are determinedby the filter coefficients selected in accordance with the leveldetermined by the level determination unit 104, and a difference betweenthe output of an adder 101 and an output signal of a quantizer 107 isinput to the variable filter 106 and added to the m-bit input signal.

[0009] However, there are the following problems in the conventionaldelta-sigma modulator. In order to raise the S/N ratio, it must beensured that the amount of noise shaping is increased using a(high-order) delta-sigma modulator of a high-order circuitconfiguration. However, in a high-order delta-sigma modulator, thesystem becomes unstable when the amplitude of the input signal becomeslarge, and sometimes oscillation occurs. As a result, a balance betweena high S/N ratio and stabilization of the system cannot be achieved.

[0010] Thus, in a case where the filter coefficients are fixed, themaximum amplitude of the input signal must be restricted when ahigh-order delta-sigma modulator is used in order to raise the S/Nratio. As a result, it becomes necessary to raise the voltage at thepower switching stage in order to obtain a predetermined output, whichis a disadvantage in terms of performance and costs. Conversely, becausea low-order delta-sigma modulator must be used in order to allow aninput signal of a large amplitude, it is difficult to raise the S/Nratio.

[0011] Also, in the conventional example shown in FIG. 2, thedelta-sigma modulator is configured so that an average value of theabsolute value of the amplitude of the input signal in a pastpredetermined period of time is detected, the level of this value isdetermined, and switching is conducted by the level. However, there areproblems in that, because that which is detected is a past level, thereis a great potential for the level to have already changed before thefilter coefficients are switched, and it is difficult to achieve abalance between a high S/N ratio and stability of the system. Theseproblems can be listed as examples among the problems that the presentinvention attempts to solve.

SUMMARY OF THE INVENTION

[0012] According to a first aspect of the invention, a delta-sigmamodulator shifts quantization noise of a digital audio signal to ahigh-frequency band to reduce noise of the band of the digital audiosignal. The delta-sigma modulator includes a quantization bit ratedetecting unit configured to detect a quantization bit rate of thedigital audio signal, a volume setting value detecting unit configuredto detect a volume setting value of the digital audio signal, afiltering unit configured to have plural sets of filter coefficientshaving different shaping properties and allow the quantization noise topass, and a filter coefficient switching unit configured to switch thefilter coefficients of the filtering unit in accordance with thedetection result of the quantization bit rate detecting unit and thedetection result of the volume setting value detecting unit.

[0013] According to a second aspect of the invention, a method ofswitching a delta-sigma modulator, includes detecting a quantization bitrate of a digital audio signal, detecting a volume setting value of thedigital audio signal, and switching filter coefficients in accordancewith the quantization bit rate and the volume setting value.

[0014] According to a third aspect of the invention, a digital amplifierincludes the delta-sigma modulator as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a block diagram showing the configuration of aconventional quintic delta-sigma modulator.

[0016]FIG. 2 is a block diagram showing a conventional delta-sigmamodulator described in Patent Document 1.

[0017]FIG. 3 is a functional block diagram showing a digital amplifieraccording to an embodiment of the invention.

[0018]FIG. 4 is a block diagram showing an example of a delta-sigmamodulation circuit configuring a delta-sigma modulator according to theembodiment of the invention.

[0019]FIG. 5 is a chart showing noise shaping properties of thedelta-sigma modulation circuit of FIG. 4.

[0020]FIG. 6 is a block diagram showing the configuration of thedelta-sigma modulator according to the embodiment of the invention.

[0021]FIG. 7 is a graph showing a threshold and a filter switching pointin time when a volume setting rises.

[0022]FIG. 8 is a graph showing a threshold and a filter switching pointin time when the volume setting falls.

[0023]FIG. 9 is a block diagram showing another example of a delta-sigmamodulation circuit configuring the delta-sigma modulator according tothe embodiment of the invention.

[0024]FIG. 10 is a block diagram showing yet another example of adelta-sigma modulation circuit configuring the delta-sigma modulatoraccording to the embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] An embodiment according to the invention will be described indetail below on the basis of the drawings.

[0026]FIG. 3 is a functional block diagram of a digital amplifier usinga delta-sigma modulator according to the embodiment of the invention.

[0027] In FIG. 3, a digital amplifier 20 using the delta-sigma modulatoraccording to the embodiment of the invention includes: a digitalinterface 1 that acquires an input signal from a digital source such asa CD (Compact Disc) or a DVD (Digital Versatile Disc); an over-samplingcircuit 2 that over-samples the input signal acquired from the digitalsource via the digital interface 1; a digital volume 3 that adjusts gainwith respect to the input signal whose sampling frequency has beenraised; a delta-sigma modulator 4 that reduces a quantization bit ratewith respect to the gain-adjusted input signal; a PCM (Pulse CodeModulation)/PWM (Pulse Width Modulation) convertor 5 that converts apulse code modulation signal obtained by the delta-sigma modulator 4into a pulse width modulation signal; a power switch 6 that switches anunillustrated power source using the period of the pulse widthmodulation signal, amplifies the amplitude of the pulse width modulationsignal and generates a current applied to a speaker 8; and a low-passfilter 7 that removes the high-band component included in the generatedcurrent.

[0028] Next, the delta-sigma modulator according to the embodiment ofthe invention will be described.

[0029] First, an example of a delta-sigma modulation circuit making upthe delta-sigma modulator 4 according to the embodiment of the inventionis shown in FIG. 4, and the basic configuration and operation thereofwill be described.

[0030] As shown in FIG. 4, the delta-sigma modulation circuit includesan adder Σ, a filter and a quantizer. In FIG. 4, quantization noiseNq(z) is fed back to an input side via the filter H(z). As a result, thespectrum of the quantization noise is transformed by the followingequation.

Nq′(z)=(1−H(z))Nq(z)  (1)

[0031] Here, for example, when a 24-bit signal of 768 kHz is input, a6-bit output signal Y(z) is output. Ordinarily, in the digitalamplifier, the filter H(z) is appropriately designed and, as shown inthe noise shaping property graph shown in FIG. 5, the quantization noiseis shifted to a high-frequency band, whereby the noise of the audio bandis reduced and the S/N ratio is secured.

[0032] The delta-sigma modulator 4 according to the embodiment of theinvention is configured as shown in FIG. 6 using a quintic delta-sigmamodulation circuit (e.g., the circuit shown in FIG. 4).

[0033] As shown in FIG. 6, the delta-sigma modulator 4 includes aquantization bit rate detection unit 9 that detects the quantization bitrate of a digital audio signal that is the input source; a volumesetting value detection unit 10 that detects a volume setting value ofthe digital audio signal; a filter 11 that includes plural sets offilter coefficients having different shaping properties and allows thequantization noise to pass; and a filter coefficient switcher 17 thatswitches the filter coefficients of gain adjustors 12 to 16 of thefilter 11 in accordance with the bit rate and the volume setting valueof the digital audio signal.

[0034] In this manner, because the delta-sigma modulator 4 according tothe embodiment of the invention includes a quintic delta-sigmamodulation circuit, the number of coefficients for each of delta-sigmaconversion circuits is five. Additionally, the delta-sigma modulator 4is characterized in that it plurally has filter coefficients where thefive coefficients are used as one set.

[0035] That is, as shown in FIG. 6, the delta-sigma modulator 4 has twosets of filter coefficients: quintic (high-order) filter coefficients(a1, a2, a3, a4, a5) and tertiary (low-order) filter coefficients (b1,b2, b3, 0, 0). In this manner, a tertiary (low-order) delta-sigmaconverter is realized by the tertiary (low-order) filter coefficientshaving two 0 coefficients.

[0036] As a specific example of the quintic and tertiary filtercoefficients used here, an example such as the following numericalvalues can be given.

[0037] Quintic filter coefficients: (−5, 10, −10, 5, −1)

[0038] Tertiary filter coefficients: (−3, 3, −1, 0, 0) The filtercoefficient switcher 17 always switches the filter coefficients to thetertiary filter coefficient set (b1, b2, b3, 0, 0) in the case of asource having a low quantization bit rate such as a CD, and in principleswitches the filter coefficients to the quintic filter coefficients (a1,a2, a3, a4, a5) in the case of a source having a high quantization bitrate such as a DVD. However, the tertiary filter coefficients are usedin a case where the volume setting value is set higher than a thresholdvalue, which is lower than the maximum value thereof by a predeterminedvalue, even in the case of a source with a high quantization bit rate.

[0039] At the time that the volume setting value rises, as in FIG. 7that shows the threshold value and the point in time where the filtercoefficients are switched, the filter coefficients are switched fromquintic (high-order) to tertiary (low-order) when the volume settingvalue that had been set lower than the predetermined threshold value ischanged to a value higher than the threshold value.

[0040] Conversely, at the time that the volume setting value falls, asin FIG. 8 that shows the threshold value and the point in time where thefilter coefficients are switched, the filter coefficients are switchedfrom tertiary (low-order) to quintic (high-order) when the volumesetting value that had been set higher than the predetermined thresholdis changed to a value lower than the threshold.

[0041] Assuming, for example, that the maximum value of the volumesetting is 0 dB, the threshold shown in FIGS. 7 and 8 is preferablyabout −6 dB. The reason for this is because oscillation occurs when theinput amplitude is not held to about 60% or less of the maximum value ofthe volume setting value with the quintic filter coefficients.

[0042] Also, when the filter coefficients are switched from tertiary toquintic, the filter coefficient switcher 17 resets memories 18 to 22storing the quantization noise to zero.

[0043] The reason for this is to prevent the quantization noisefrombeing excessively fed back when the filter coefficients are switchedfrom the tertiary filter coefficient set to the quintic filtercoefficient set, because the numerical values of the quintic filtercoefficients are larger than those of the tertiary filter coefficients,as in the specific example of the numerical values mentioned above.

[0044] It should be noted that it is not necessary to reset the memorieswhen the filter coefficients are switched from quintic to tertiary.However, because a control program is required in order to ensure thatthe memories are not reset, here, the memories may be reset in order tosimplify the overall control program.

[0045] Moreover, when the gain rises from a current volume setting valueshown in FIG. 7 to a new volume setting value, the filter coefficientsare switched from quintic (high-order) to tertiary (low-order) near thepoint in time when the rising starts. When the gain falls from thecurrent volume setting shown in FIG. 8 to a new volume setting, thefilter coefficients are switched from tertiary (low-order) to quintic(high-order) near the point in time when the falling ends.

[0046] By switching the filter coefficients near the point in time whenthe rising starts and near the point in time when the falling ends, thehigh-order filter coefficient set is always used with a volume settingvalue that is lower than the threshold (predetermined value). Thus, thesystem can be prevented from becoming unstable.

[0047] Next, the operation of the digital amplifier according to thepresent embodiment will be described.

[0048] After the over-sampling circuit 2 has raised the samplingfrequency of the input signal from a digital source such as a CD or aDVD acquired via the digital interface 1, the digital volume 3 adjuststhe gain.

[0049] Then, the delta-sigma modulator 4 reduces the quantization bitrate of the gain-adjusted input signal. After the pulse code modulationsignal obtained by the delta-sigma modulator 4 has been converted intothe pulse width modulation signal by the PCM/PWM converter 5, theunillustrated power source is switched by the power switch 6 using theperiod of the pulse width modulation signal, the amplitude of the pulsewidth modulation signal is amplified, and the current applied to thespeaker 8 is generated. After the high-frequency component of thegenerated current has been removed by the low-pass filter 7, the currentis applied to the speaker 8.

[0050] Then, in the delta-sigma modulator 4, the filter coefficients areswitched to the tertiary filter coefficient set in the case of a sourcewhose input signal has a low quantization bit rate and switched to thequintic filter coefficient set in the case of a source having a highquantization bit rate.

[0051] In the case of a source having a high quantization bit rate, thefilter coefficients are switched from quintic to tertiary when thevolume setting value of the digital volume 3 has been changed to a valueof about 50% or more of the maximum value thereof.

[0052] Also, the memories storing the quantization noise are reset tozero when the filter coefficients are switched from tertiary to quintic.

[0053] In this manner, the delta-sigma modulator 4 detects thequantization bit rate of the input signal (quantization bit ratedetection step), switches the filter coefficients to the tertiary filtercoefficient set in the case of a source having a low quantization bitrate, and switches the filter coefficients to the quintic filtercoefficient set in the case of a source having a high quantization bitrate (filter coefficient switching step). Thus, with respect to a sourcehaving a low quantization bit rate, the S/N ratio that the source hascan be secured even with a tertiary delta-sigma modulator, and thesystem does not become unstable even if the amplitude of the inputsignal becomes large. Additionally, with respect to a source having ahigh quantization rate, the S/N ratio can be set high by the delta-sigmamodulator 4 operating as a quintic delta-sigma modulator.

[0054] Also, even in the case of a source having a high quantization bitrate, the delta-sigma modulator 4 detects the volume setting value(volume setting detecting step) and switches the filter coefficientsfrom quintic to tertiary when the value thereof has been changed to ahigh value of 50% or more of the maximum value (filter coefficientswitching step). Thus, because the high-order filter coefficient set isalways used with a volume setting that is lower than the threshold, thesystem can be prevented from becoming unstable.

[0055] It should be noted that, because it is rare for the volume valueto approach the maximum value, a high S/N ratio can be secured in anordinary state of use.

[0056] Also, the memories storing the quantization noise are reset tozero when the filter coefficients are switched to the high-ordercoefficients. Thus, the quantization noise can be prevented from beingexcessively fed back even when the filter coefficients are switched fromthe tertiary filter coefficient set to the quintic filter coefficientset.

[0057] As other examples of the delta-sigma modulation circuit, thereare the circuits shown in FIGS. 9 and 10. In the delta-sigma modulationcircuits shown in these diagrams, not only the quantization noise butthe signal have transfer functions. Thus, although the frequencyproperties of the signals are not flat, filter coefficient switching canalso be applied to delta-sigma modulation circuits of suchconfigurations as in the delta-sigma modulator according to the presentembodiment.

[0058] As described in detail above, the delta-sigma modulator accordingto the present embodiment is a delta-sigma modulator that shiftsquantization noise of a digital audio signal to a high-frequency band tothereby reduce noise of a band of the digital audio signal. Thedelta-sigma modulator includes the quantization bit rate detection unit9 that detects the quantization bit rate of the digital audio signal;the volume setting value detection unit 10 that detects the volumesetting value of the digital audio signal; the filter 11 that includesplural sets of filter coefficients having different shaping propertiesand allows the quantization noise to pass; and the filter coefficientswitcher 17 that switches the filter coefficients of the filter 11 inaccordance with the detection result of the quantization bit ratedetection unit 9 and the detection result of the volume setting valuedetection unit 10. Thereby, the delta-sigma modulator can switch thefilter coefficients of the filter 11 in accordance with the detectedvalue of the quantization bit rate of the input signal and the volumesetting value. Thus, the delta-sigma modulator can switch the filtercoefficients to the low-order coefficient set in the case of an inputsignal having a low quantization bit rate and can switch the filtercoefficients to the high-order coefficient set in the case of an inputsignal having a high quantization bit rate. The system does not becomeunstable even if the amplitude of the input signal becomes large. Also,with respect to an input signal having a high quantization bit rate, theS/N ratio can be set high because the delta-sigma modulator operates asa high-order delta-sigma modulator.

[0059] Also, the method of switching the delta-sigma modulator accordingto the present embodiment includes the quantization bit rate detectingstep that detects the quantization bit rate of a digital audio signal;the volume setting detecting step that detects the volume setting valueof the digital audio signal; and the filter coefficient switching stepthat switches the filter coefficients in accordance with thequantization bit rate and the volume setting value. Thus, the filtercoefficients can be switched to the low-order coefficient set in thecase of an input signal having a low quantization bit rate, and thefilter coefficients can be switched to the high-order coefficient set inthe case of an input signal having a high quantization bit rate. Thesystem can be prevented from becoming unstable even if the amplitude ofthe input signal becomes large. Also, with respect to an input signalhaving a high quantization bit rate, the S/N ratio can be set high.

[0060] Also, because the digital amplifier according to the presentembodiment includes the delta-sigma modulator of the present embodiment,a balance between a high S/N ratio and stability can be achieved.

What is claimed is:
 1. A delta-sigma modulator that shifts quantizationnoise of a digital audio signal to a high-frequency band to reduce noiseof the band of the digital audio signal, the delta-sigma modulatorcomprising: a quantization bit rate detecting unit configured to detecta quantization bit rate of the digital audio signal; a volume settingvalue detecting unit configured to detect a volume setting value of thedigital audio signal; a filtering unit configured to have plural sets offilter coefficients having different shaping properties and allow thequantization noise to pass; and a filter coefficient switching unitconfigured to switch the filter coefficients of the filtering unit inaccordance with the detection result of the quantization bit ratedetecting unit and the detection result of the volume setting valuedetecting unit.
 2. The delta-sigma modulator according to claim 1,wherein: the filter coefficients include a first coefficient set and asecond coefficient set; an order of the first coefficient set is lowerthan that of the second coefficient set; and the filter coefficientswitching unit switches the filter coefficients to the first coefficientset when the quantization bit rate is equal to or less than apredetermined number.
 3. The delta-sigma modulator according to claim 1,wherein: the filter coefficients include a first coefficient set and asecond coefficient set; an order of the first coefficient set is lowerthan that of the second coefficient set; the filter coefficientswitching unit switches the filter coefficients to the first coefficientset when the quantization bit rate is equal to or greater than apredetermined number and the volume setting value is equal to or greaterthan a predetermined value; and the filter coefficient switching unitswitches the filter coefficients to the second filter coefficient setwhen the quantization bit rate is equal to or greater than thepredetermined number and the volume setting value is less than thepredetermined value.
 4. The delta-sigma modulator according to claim 1,wherein: the filtering unit includes a memory, which stores thequantization noise; and when the filter coefficient switching unitswitches the filter coefficients, contents of the memory are reset. 5.The delta-sigma modulator according to claim 1, wherein: the filteringunit includes a memory, which stores the quantization noise; and whenthe filter coefficient switching unit switches the filter coefficientsso that an order thereof becomes larger, contents of the memory arereset.
 6. The delta-sigma modulator according to claim 1, wherein: whenthe volume setting value is changed and gain rises, the filtercoefficient switching unit switches the filter coefficients near thepoint in time when the rising starts; and when the volume setting valueis changed and gain falls, the filter coefficient switching unitswitches the filter coefficients near the point in time when the fallingends.
 7. A method of switching a delta-sigma modulator, the methodcomprising: detecting a quantization bit rate of a digital audio signal;detecting a volume setting value of the digital audio signal; andswitching filter coefficients in accordance with the quantization bitrate and the volume setting value.
 8. A digital amplifier comprising adelta-sigma modulator that shifts quantization noise of a digital audiosignal to a high-frequency band to reduce noise of the band of thedigital audio signal, wherein the delta-sigma modulator includes: aquantization bit rate detecting unit configured to detect a quantizationbit rate of the digital audio signal; a volume setting value detectingunit configured to detect a volume setting value of the digital audiosignal; a filtering unit configured to have plural sets of filtercoefficients having different shaping properties and allow thequantization noise to pass; and a filter coefficient switching unitconfigured to switch the filter coefficients of the filtering unit inaccordance with the detection result of the quantization bit ratedetecting unit and the detection result of the volume setting valuedetecting unit.